Method of removing PECVD residues of fluorinated plasma using in-situ H2 plasma

ABSTRACT

In a method of affecting cleaning or chamber process control to remove residues of fluorinated discharges from internal PECVD chamber hardware during manufacture of a semiconductor or integrated circuit, the improvement of removing the fluorinated discharges without opening the chamber and without causing chamber downtime, comprising:  
     a) maximizing H-atom concentration in a gas mix of a plasma containing H 2  through the use of high rf power and low pressure to obtain an in-situ H 2  plasma; and  
     b) subjecting a reactor chamber containing build-up residues from previous chamber treatment with a fluorinated plasma, with the in-situ H 2  plasma from step a) without opening the chamber and without shutting down the chamber to remove the build-up residues of the fluorinated plasma.

FIELD OF THE INVENTION

[0001] The present invention relates to a method of removing plasmaenhanced chemical vapor deposition (PECVD) residues from a reactionchamber during a process of forming semiconductors or integratedcircuits. These residues result from fluorinated discharges used toremove unwanted film deposition from PECVD chamber walls when preparingsemiconductor or integrated circuits.

BACKGROUND OF THE INVENTION

[0002] It is known that manufacturing semiconductor devices on a waferentails a number of steps, inclusive of photolithography and etching,thin film deposition, and the use of ion implantation steps alternatelyperformed in order to develop or build-up the semiconductor device orwafer. Typically, the photolithography steps include coating a waferwith a photoresist wherein an ultraviolet photosensitive organicmaterial is utilized. The photoresist is exposed through a mask, afterwhich the resist is developed. Next, the exposed photoresist is etchedthereby leaving given exposed areas on the surface of the wafer.Following the foregoing steps, additional processing steps such asdeposition, implantation or etching may be employed on the exposedareas.

[0003] In deposition processes, particularly in PECVD processes, it isnecessary to periodically remove or clean deposition material from thechamber or reactor hardware. A fluorine-based plasma discharge iscommonly used to remove dielectric material such as silicon dioxide(SiO₂), silicon nitride (Si₃N₄), and silicon oxynitrides (SiON). Duringsuch fluorinated plasma cleaning, AlF₃ (which is a by-product of thecleaning) grows on exposed internal PECVD chamber hardware by reactionof various fluorine species with the aluminum or aluminum based chamberparts. This accumulation of AlF₃ on the chamber hardware alters theensuing plasma chemistry and adversely impacts film depositionproperties. In essence, the slow accumulation of AlF₃ causes driftingfilm properties and process control problems. (Refer to publication byB. Smith, et al, J. Electrochem. Soc. accepted for November 2001.)

[0004] Eventually, the AlF₃ must be removed in order to maintain filmproperties within some process window. Wet chemical cleaning is commonlyused to restore PECVD chamber performance (since AlF₃ is water soluble).However, wet cleaning unfortunately involves removing the PECVD chamberfrom operation, disassembling the chamber, and cleaning the chamberparts in a wet chemical bath. This wet chemical cleaning unfortunatelycreates considerable chamber downtime during the wet cleaning procedure.

[0005] In the past, H₂ plasma etching of AlF₃ films on wafers has beendemonstrated by S. G. Pearton, et al, Mat. Res. Soc. Symp. Aoc., Volume282 (1993), p. 131. Pearton et al used H₂ plasma to remove AlF₃etch-stop layers in GaAs-based wafer processing.

[0006] U.S. Pat. No. 5,882,489 disclose processes for cleaning andstripping photoresist from surfaces of semiconductor wafers. The processentails ashing the organic resist from a device, rinsing the device inwater, and sputtering the rinsed device to remove residual contaminants.The stripping step is a dry etching process such as a microwavedownstream process, a RIE process, or sequential or simultaneousmicrowave downstream and RIE process, wherein the rinsing step isperformed with deionized (DI) water, and the sputtering step isperformed with argon. The process is especially useful for etching viaholes when the holes penetrate a conductive layer and create insoluble,inorganic contaminants such as AlF₃.

[0007] A method for etch rate enhancement by background oxygen controlin a soft etch system is disclosed in U.S. Pat. No. 6,143,144. Thissputter etch cleaning process to remove or sputter off particles fromthe substrate surface within the processing chamber is accomplished by:

[0008] positioning a first substrate to be processed within a processingchamber, the first substrate including a material layer containingoxygen;

[0009] introducing a process gas into the chamber;

[0010] inductively coupling electrical energy to the process gas in thechamber to form an ionized gas plasma in the chamber;

[0011] positioning a second material substrate proximate the firstsubstrate in the processing chamber;

[0012] biasing the first and second substrates with RF electrical energyso that the plasma etches the first substrate material layer and thesecond substrate, the material etched from the first substrate materiallayer producing activated oxygen in the gas plasma;

[0013] the second substrate being formed of a material which reacts withactivated oxygen to form a stable oxygen-containing compound such thatmaterial etched from the second substrate reduces activated oxygen inthe gas plasma;

[0014] whereby residual oxygen in the processing chamber is reduced tomaintain an etch rate for subsequent sputter etching processes.

[0015] U.S. Pat. No. 5,017,403 disclose the use of plasma-enhancedchemical vapor deposition (PECVD) to form dielectric films. The processentails:

[0016] (a) providing a substrate in a chamber;

[0017] (b) flowing a reactant gas in the chamber;

[0018] (c) generating a plasma between the electrodes by R.F. power todissociate the gas and deposit a predetermined planarization layer ofcarbonaceous material on the substrate; while maintaining the substrateat a relatively low temperature wherein the layer is soft as depositedand is then hardened by thermal or plasma treatment.

[0019] A low pressure and low power Cl₂/HCl process for sub-micron metaletching is disclosed in U.S. Pat. No. 5,976,986. The method entails:

[0020] placing aluminum metallization coated on at least one surfacewith a barrier layer in an etch chamber;

[0021] creating a transformer coupled plasma from Cl₂, HCl, and an inertgas within the etch chamber, without a magnetic field, using separatelypowered electrodes positioned above and below aluminum metallizationswherein each of the electrodes are powered by less than 350 Watts, andwherein the pressure in the etch chamber is less than 15 milliTorr;

[0022] etching the aluminum metallizations with ions and radicals formedin the plasma; and adjusting a concentration of the Cl₂ in the plasmaduring the creating and etching steps between a first higherconcentration during etching of the aluminum metallization and a secondlower concentration during etching of the barrier layer.

[0023] The addition of hydrogen to plasma is used to reduce corrosionduring the etching of aluminum layers.

[0024] In the art of PECVD processing where fluorinated plasmadischarges are utilized, there is a need to remove unwanted residuesfrom PECVD chamber hardware without wet cleaning the chamber (i.e.,disassembling the chamber and cleaning the chamber parts in a wetchemical bath), which results in considerable chamber downtime.

SUMMARY OF THE INVENTION

[0025] One object of the present invention is to provide an improvedmethod for removing PECVD chamber residues resulting from fluorinatedplasma cleaning during a process for forming semiconductors orintegrated circuits.

[0026] Another object of the present invention is to provide a methodfor removing PECVD chamber residues of AlF₃ or derivatives thereof thatalter plasma chemistry and adversely impacts film deposition properties.

[0027] A further object of the present invention is to provide a methodfor removing PECVD chamber residues resulting from fluorinated plasmacleaning; namely, AlF₃ or its derivatives, by utilizing in-situ H₂plasma or mixtures containing H₂, and thereby avoiding wet chemicalcleaning which requires disassembling the chamber, cleaning the chamberparts in wet chemical baths, and incurring considerable chamber downtimeduring the wet chemical cleaning procedure.

[0028] A further object yet still of the present invention is to providea method for removing PECVD chamber residues resulting from fluorinatedplasma cleaning by using in-situ H₂ plasma to restore chamberperformance without opening the chamber, and thereby increasing toolavailability and freeing maintenance resources.

[0029] In general, the invention process of using H₂ plasma to etch AlF₃accumulated in the reaction chamber and to improve control over filmproperties is accomplished after a fluorinated plasma clean by:

[0030] subjecting the reaction chamber to a mixture of He/H₂ at a flowrate of 1,000/200 sccm (standard cubic centimeters per minute), at about750 W rf, at about 0.8 Torr for about 5 seconds to strike the plasma,and then cleaning the chamber by subjecting it to H₂ at about 500 sccmfor 5-60 seconds, at about 500 W rf, at about 0.5 Torr.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

[0031]FIG. 1 is a graph showing film thickness U % versus number ofwafers processed, and reflects drifting film properties in a PECVDchamber between wet cleans (denoted WC).

[0032]FIG. 2 depicts a reaction chamber after one or more fluorinatedplasma cleans and shows AlF₃ build-up on the powered electrode.

[0033]FIG. 3A is a graph generally showing SiON film thickness versus rfpower (DARC is a specific SiON application), consistent with theaccumulation of AlF₃ on a shower head to affect power coupling intoplasma.

[0034]FIG. 3B is a graph generally depicting SiON optical constantsrefractive index (n) and extinction coefficient (k) versus rf power,which is the primary knob for adjusting plasma density. The changes thatare induced by reducing power are consistent with the film driftproblem.

[0035]FIG. 4A is a graph depicting change in SiON film thickness bylocation across a wafer after processing several thousand wafers andrepresents the impact of non-uniform AlF₃ build-up (concentrated incenter), and provides some indication as to why the U % drifts.

[0036]FIG. 4B is a graph showing the change in optical properties, n andk, by location across a wafer after processing several thousand wafersdue to non-uniform AlF₃ build-up (concentrated in the center), wherethere is diminished plasma density in the center of the chamber.

[0037]FIG. 5A is a graph comparing the number of wafers processedbetween wet cleans (performed as necessary to control SiON filmthickness U %) for processes with and without in-situ H₂ plasma.

[0038]FIG. 5B is a graph showing drift in SiON thickness U %. The rateof drift is greatly reduced after introducing H₂ plasma to remove AlF₃residues.

[0039]FIG. 6 is a graph generally showing the influence of rf power onH-atom concentration in a plasma as predicted by actinometry for amixture of 99% H₂ and 1% Ar.

[0040]FIG. 7 is a graph generally showing the effect of pressure onH-atom concentration in a plasma as predicted by actinometry for amixture of 99% H₂ and 1% Ar.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT OF INVENTION

[0041] In PECVD processing, a fluorine containing discharge (here as amixture of CF₄+N₂O) is introduced periodically into the chamber toremove excess deposition material (e.g., SiO₂ or Si₃N₄) in order tomaintain consistent film properties from wafer to wafer. The fluorinateddischarge leaves a light residue in the chamber that accumulates oversuccessive treatments. The build-up in the chamber was identified asAlF_(x) by EDX (energy dispersive x-ray analysis).

[0042] Accumulation of fluorine discharge residue gradually affects filmdeposition properties. An example of said drifting film properties in aPECVD chamber is shown in the graph of FIG. 1 which shows SiON filmthickness uniformity (U %) versus the number of wafers processed. Ingeneral, all film properties including film thickness, thickness U %, n,k, and stress are subject to gradual drift depending on the sensitivityof the particular process and process parameter. Wet cleans (denoted WCin FIG. 1) are invasive procedures typically used to remove chamberresidue and restore film properties. After the wet clean (WC), the SiONfilm thickness profile is relatively flat. With use, the profile becomescenter thin and U % increases. U % is governed by the formula:${U\quad \%} = {100 \cdot \frac{s}{\overset{\_}{x}}}$

[0043] where s is the standard deviation of film thickness measured atnine sites on a wafer and {overscore (x)} is the mean of the filmthickness measured at the same nine sites.

[0044] The build-up of AlF₃ on the powered electrode (typically theshowerhead in commercial reactors) may be seen in the parallel plateplasma reactor diagram of FIG. 2. The AlF₃ is a dielectric that inhibitsthe coupling of rf power into the plasma, resulting in reduced plasmadensity. An evenly distributed AlF₃ build-up affects the plasmachemistry which ultimately determines the properties of the filmdeposited. FIGS. 3A and 3B show the impact of plasma density on filmproperties as rf power directly modulates plasma density (i.e., higherrf power implies higher plasma density). Because the transfer of energyis affected uniformly across the chamber, film properties are impactedin a roughly uniform manner.

[0045] In practice the fluorinated discharge that produces the AlF₃residue may not be perfectly uniform. An uneven distribution of AlF₃ (orother derivatives from fluorinated plasma cleaning) on the showerheadcauses an uneven transfer of power into the plasma, thereby leading touneven plasma density across the chamber. Since plasma density plays aprominent role in the reaction chemistry, varying the plasma densitylocally can yield locally different film properties, as may be seen fromFIGS. 4A and 4B, which shows respectively, a change in thickness versuslocation on a wafer and a change in n and k versus location on a waferduring the AlF₃ build-up. In FIG. 4A, the thickness versus positionrepresents film properties over several thousand wafers due tonon-uniform AlF₃ build-up (concentrated in the center), and therebyprovides some indication as to why the U % drifts. In FIG. 4B, there isalso shown the change in n and k versus position over several thousandwafers due to non-uniform AlF₃ build-up (concentrated in the center)where there is diminished plasma density in the center of the chamber.

[0046] This phenomenon is the same for oxide and nitride films, but to aslightly lesser degree.

[0047] Wet cleaning PECVD chambers is typically performed to remove AlF₃residue and restore (i.e., control) film properties. Wet cleanstemporarily remove a chamber from operation and require maintenanceresources for disassembling the chamber, initiating the wet chemicalclean, and eventually reassembling the chamber. In large scalesemiconductor manufacturing where a large number of PECVD chambers areemployed, frequent chamber wet cleans can easily impact overallmanufacturing productivity by reducing chamber availability andconsuming maintenance resources. The invention process uses in-situ H₂plasma to reduce or remove fluorinated discharge residues, namely AlF₃,in a less invasive manner. In essence, the invention process of removingthis build-up from fluorinated plasma cleaning by using in-situ H₂plasma eliminates or reduces the need for wet chemical cleaning, therebyimproving chamber availability and freeing maintenance resources. Theimprovement realized by employing in-situ H₂ plasma is illustrated inFIG. 5A which compares the number of wafers processed between SiONthickness U % failures with and without H₂ plasma cleaning. FIG. 5Bshows SiON thickness U % over time and reflects a significant reductionin U % drift rate after beginning H₂ plasma treatments. The inventionprocess is the first to utilize H₂ plasma etching of AlF₃ to controlchamber conditions in order to enhance control over PECVD processes andthe properties of the deposited film. The invention process is the firstin-situ chemical process for removal of fluorinated discharge residues.As such, the H₂ plasma process has distinct advantages over the wetclean process typically used in terms of chamber productivity and use ofmaintenance resources.

[0048] While the preferred embodiment of the invention applies the H₂plasma cleaning in-situ, it should be noted that an ex-situ processwould also suffice.

[0049] Hydrogen atoms generated in H₂ plasma are responsible for removalof AlF₃ by reduction reaction. Consequently, it is desirable to maximizethe H-atom concentration (denoted [H]) for efficient removal of AlF₃residues. From FIG. 6, it can be seen that the high rf power isconducive to high hydrogen atom concentration. The use of opticalemission spectroscopy (OES) and actinometry, a related technique, isutilized to identify process conditions that maximize H-atomconcentration in the plasma based on the relative emission from excitedH and Ar atoms.

[0050] Referring to FIG. 7, it may also be seen that low pressure isalso conducive to high [H]. Therefore, it is clear from FIGS. 6 and 7that the combination of low pressure and high power are togetherconducive to high [H]. In general, the H-atom concentration in theplasma may be determined by the following formula:$\frac{{Intensity}\quad {of}\quad H}{{Intensity}\quad {of}\quad {Ar}} \cdot {\alpha \quad\lbrack {H\quad {atom}} \rbrack}$

[0051] By use of H₂ plasma to etch AlF₃ on the faceplate of the chamber,improved control over film properties was obtained. This in-situprocedure effectively eliminates the need for wet cleans and increasestool availability. Restoration of chamber performance by removing AlF₃removes AlF₃ growth on the chamber wall that alters PECVD chamberperformance, and the invention process for removing the AlF₃ is superiorto removal by wet chemical or mechanical means, as these means requireconsiderable chamber downtime. Further, the in-situ H₂ plasma removesAlF₃ without opening the chamber, thereby increasing tool availabilityand freeing maintenance resources.

[0052] While the invention has been described in terms of its preferredembodiments, those skilled in the art will appreciate that the inventionmay be practiced with modifications within the spirit and scope of theappended claims.

1.-7. Cancelled
 8. In a method of affecting cleaning to remove AlF₃residue from walls of a reactor chamber, the method comprising the stepsof: a) identifying process conditions that maximize H-atom concentrationin a plasma of a gas mixture containing H₂ and Ar using optical emissionspectroscopy to identify the H atom concentration in the plasma based onthe relative emission intensity from excited H and Ar atoms by theformula:$\frac{{intensity}\quad {of}\quad H}{{intensity}\quad {of}\quad {Ar}} \sim {H\quad {atom}\quad {{concentration}.}}$

b) subjecting said reactor chamber in situ to H₂ gas or a gas mixture ofHe/H₂ according to the process conditions identified in step a) withoutopening said chamber and without shutting down said chamber to affectreduction and removal of said AlF₃ residue.
 9. Cancelled
 10. A method ofcleaning a chamber, the method comprising: determining cleaning processconditions that maximizes H atom concentration in the chamber; injectinginto the chamber a first gas mixture in accordance with striking processconditions; striking the first gas mixture, thereby creating a firstplasma; and injecting into the chamber a second gas mixture inaccordance with the cleaning process conditions, wherein the cleaningprocess conditions are different than the striking process conditions.11. The method of claim 10, wherein the cleaning process conditionsincludes one or more of a flow rate, a pressure, and an RF power. 12.The method of claim 10, wherein the step of striking the first gasmixture is performed at a flow rate of about 1,000/200 sccm, at apressure of about 0.8 Torr, and at an RF power of about 750 W for about5 seconds.
 13. The method of claim 10, wherein the chamber remainsclosed.
 14. The method of claim 10, wherein the cleaning processconditions are determined to be a flow rate of about 500 sccm, an RFpower of about 500 W, and a pressure of about 0.5 Torr.
 15. The methodof claim 10, wherein the step of determining cleaning process conditionsis performed by using optical emission spectroscopy with an Ar tracer todetermine the H atom concentration, the H atom concentration beingdetermined by the formula:$\frac{{intensity}\quad {of}\quad H}{{intensity}\quad {of}\quad {Ar}} \sim {H\quad {atom}\quad {{concentration}.}}$


16. The method of claim 10 wherein the first gas mixture comprises amixture of He and H₂.
 17. The method of claim 10 wherein the second gasmixture comprises a mixture of Ar and H₂.